The present invention relates to a hydraulic auxiliary power steering system for vehicles, especially heavy motor vehicles but also for marine craft.
Hydraulic auxiliary power steering systems generally have a mechanical connection between a manual actuating mechanism such as a steering wheel and the steered parts of the vehicle or a rudder in the case of a marine craft. Additionally, hydraulic auxiliary power amplification to facilitate steering is provided. One type of such power steering system is equipped with a pressure storage reservoir. The pressure storage reservoir is ordinarily arranged to meet a high fluid operating pressure demand since substantial steering forces are required for the steering of heavy vehicles at standstill and at low speeds. For speeds below 5 km/h, pressures up to 180 BAR are required for generating these forces. Reservoirs arranged to store such pressure with reliability are relatively massive and expensive.
At higher speeds, only relatively low pressures of up to 5 BAR for example, are required for steering purposes. When the servomotor is then operated through the steering control valve from the storage reservoir charged to a pressure of 180 BAR, the 5 BAR pressure needed for operation of the servomotor is exceeded by 175 BAR resulting in a drop in pressure primarily across the steering control valve. Accordingly, relatively high leakage losses and power losses occur. This problem may be particularly dealt with by providing the steering control valve with very small clearances and finishing tolerances attainable only by very expensive finishing procedures. The high cost involved in such finishing procedures is not, however, warranted because steering activity during standstill and low speeds of travel including parking, represents only about 1% of the total time of steering activity.